The form of the furnace Is shown in Fig. 142, The arch a is 18 in. above the hearth at the lire end and only 6 in. at the chimney end. The fireplace b measures 5 ft. long by 1 1/2 ft. wide: the bridge c is 27 in. wide and 16 in. high. The hearth is occupied by a loosely 2 A fitting cast - iron pan d, 10 ft. long, 5 ft. wide, and shelving. from 9 in. deep at the chimney end to 8 in. near the fire; it has an outlet at e, closable by an iron plug. The lead is first melted in an adjacent iron pot /, heated by a separate fire g, in quantities of 8 to 10 tons at a time; when fluid, it is ladled into the gutter A, communicating with the softening pan. The softening process may be completed in a single day, or it may require several weeks, according to the amount of oxidation necessary to be accomplished. Its progress is noted by stirring up the metal through the working - door i till an iridescent film makes its appearance; also by watching for a peculiar crystalline look in samples ladled into ingot moulds at intervals. The dross formed on the surface, consisting mainly of antimony and lead oxides, is occasionally raked off, mixed finally with coal, and pulverized in an edge - runner mill, ready for smelting in a smalt reverberatory.

The resulting lead - antimony alloy is reheated with new charges till it is - fit only for typefounding.

Fr.;. 141.

Lead Part 11 300149

A much simpler softening process, adapted for small quantities, is to throw repeated doses of a mixture of soda nitrate, soda, and lime upon the skimmed surface of the metal melted in a cast - iron pan, removing the dross as formed.

Desilverizing Lead

Three methods are in use in this country, viz., Pattin - son's, Parkes', and the Rozan method. By Pattinson's process the lead is melted in a series of 8 to 12 strong hemispherical cast - iron pots set in a row; the pots are of such a size as to be capable of dealing with 6 to 10 tons of lead, the last pot (the "market pot") being, however, smaller than the others; each is heated by a separate fire beneath. The lead melted in one of these pots is allowed to cool, and as it cools, lead, in great measure free from - silver, crystallizes, and is removed by means of an iron ladle perforated with holes, through which the liquid lead strains away. The lead which remains liquid contains most of the silver. To enrich the liquid lead further, the operation is repeated in another pot, and the process is repeated - until a rich lead Is obtained, containing, say, 500 to 600 ox. of silver - to the ton. The crystals - of lead strained from the pots have also to be desilverized in a similar way. The rich silver - lead is then cupelled in a hearth made of bone - earth, where part of the lead is blown off and collected as litharge (to be subsequently reduced with carbonaceous matter in another furnace), while part is absorbed by the substance of the cupel.

The ladle measures about 18 in. wide and 5 in. deep, with holes 1/2 in. diam. and 3/4 in. apart, and is fixed to a handle about' 10 ft. long, half the length being of iron and half of wood. The lead operated upon is generally in "salmons"of 120 to 140 lb. Should it contain, say, 25 oz. silver to the ton, it may be filled into the middle pot of the series. It will then be melted; and when the whole is thoroughly liquid, the fire will be drawn, and the lead allowed to cool. As it cools, a workman keeps stirring the lead, and "slicing," or freeing from the sides, the portions setting on them. As the cooling proceeds, crystals begin to form; and when a sufficient quantity appears, a second workman withdraws the crystals from this pot by means of the perforated iron ladle, and passes them into the pot on his right, continuing to do so until he has thus moved ] of the lead. When he has done this, he withdraws with a solid - bottomed ladle the 1/3J of liquid lead remaining, and moves it to the pot on his left. The central pot is again filled with original silver - lead, and the same operation is repeated.

If this has been properly conducted, it will be found that the liquid lead removed to the left, instead of containing 25 oz. of silver per ton, as did the original lead, will now contain 50 oz. per ton; and the crystals moved toward the market pot will now only contain 12 1/2 oz. of silver per ton. It will readily be seen that, by repeating this operation successively in the various pots, the poor lead gradually becomes poorer, until it is so free from silver as no longer to pay for working; while the rich lead, on the other hand, will gradually increase in richness till its silver contents make it fit for the refinery. In addition to the removal of the silver, another advantage is gained, in that the major portion of the antimony and copper go with the silver, while another portion is removed by oxidation during the heating.

The Rozan process, the invention of Luce Fils et Rozan, Marseilles, and known also as the "steam" process, is in theory very similar to the Pattinson process, but is applicable to lead which has not been previously softened, inasmuch as it effects the softening at the same time as the desilverization. The object sought is the enrichment of the lead by crystallization. One great advantage of the process (which indeed is mainly that which commends it to the lead smelter) is that it performs by mechanical agency, and so at comparatively small cost, what is performed less perfectly and at great cost by hand labour in the Pattinson process. Lead being melted and skimmed in a hemispherical pot, is run off into a cylindrical pot or "kettle" of iron,. where some water is run over the top, and where steam is blown in below, causing great agitation during the cooling which then takes place. The lead crystallizes in the pot as the cooling proceeds, and the crust formed is broken down by hand from time to time. When the operation is believed to be completed, the enriched liquid lead is run out through a strainer by a tap - hole below into a pot sunk in the floor of the works.